#include <bits/stdc++.h>

using namespace std;
ifstream fin("carnati.in");
ofstream fout("carnati.out");
pair<int, int> x[2005];

int main()
{   int n, c, maxi=0, sum;
    fin>>n>>c;
    for(int i=1;i<=n;i++)
    fin>>x[i].first>>x[i].second;

    sort(x+1, x+1+n);
    for(int i=1;i<=n;i++)
    {sum=0;
     for(int j=1;j<=n;j++)
     {if(j==1)sum-=c;
      else sum-=c*(x[j].first-x[j-1].first);

      if(sum<0)sum=-c;
      if(x[j].second>=x[i].second)sum+=x[i].second;

      maxi=max(maxi, sum);
     }
    }

    fout<<maxi;
    return 0;
}